Most of the metallic lead has been produced in the past by reduction of lead ores and ore concentrates. The most abundant of the lead ore sources are sulfide ores, most of which contain zinc sulfide as a major impurity. These ores and ore concentrates are normally roasted to produce oxygen containing compounds such as lead oxides and lead sulfates. These lead compounds are then added to a lead blast furnace and the lead values are smelted to produce molten lead. The presence of zinc as an impurity in these ores and concentrates requires the use of more complicated processing steps and to the use of more reducing agent in order to obtain a high recovery of the lead values.
A typical innovation in the primary process for smelting lead sulfide ores and concentrates containing zinc sulfide as a major impurity is described and patented by John Lumsden in U.S. Pat. No. 3,243,283 issued Mar. 29, 1966. In the patent, Lumsden describes the operation of primary lead blast furnace wherein the sulfides are converted to oxides prior to charging in the furnace. Carbon monoxide produced from the combustion of coke reduces the lead oxides to lead near the top of the shaft. This reaction is exothermic. When zinc is present in the charge, zinc oxide is also reduced to zinc by carbon monoxide, and the zinc vapor in turn reduces lead oxide, this reaction being very exothermic. The two exothermic reactions raise the temperatures to a degree such that premature melting occurs of some of the charge components near the top of the shaft, followed by resolidification somewhat lower in the shaft, leading to bridges and hangs.
In order to overcome this difficulty, Lumsden proposes controlling the temperature in the upper portions of the shaft by the addition of steam and preheated air at the bottom of the furnace. In conventional primary smelting practice, the temperature is controlled by the addition of large quantities of returned slag, which act as a heat sink. A typical primary furnace taps an amount of slag at the bottom in excess of the amount of lead bullion produced. Lumsden teaches techniques for the control of steam addition in such a way as to prevent hangs even with the use of less return slag. He further suggests that the use of at least 4% steam in the air blast preheated to at least 200.degree. C. is needed to arrive at the benefits of reduced hangs and decreased coke consumption.
Since one of the largest uses of lead is in the battery industry, the recovery and reuse of lead from batteries and battery scrap is an absolute necessity. One such process for recovery and reuse of lead is called a secondary lead smelting or blast furnace operation.
In contrast to primary lead smelting, the secondary lead blast furnace uses a charge containing substantial quantities of sulfur compounds, such as metal sulfides and sulfates, the chief source being the battery scrap and by-product drosses. Thus, considerable quantities of cast iron are used to reduce these sulfur compounds. Iron also reduces a portion of the lead oxide to lead. Some of the iron oxidizes to form a major constituent of the slag. Thus, iron plays a very central role in the secondary blast furnace. The charge also contains quantities of lead metal (generally alloyed with antimony) in the form of battery grids. In the secondary furnace the smelting zone appears to be lower than that postulated by Lumsden as being the case in the primary furnace.
Very small quantities of slag are added to the charge and often no slag is used at all. Typically, in a secondary blast furnace the slag tapped out of the bottom is less than 20% of the weight of the lead produced. Substantial quantities of matte are produced, however, these being predominantly composed of iron sulfides, typical quantities being 30% of the weight of lead produced.
In the instant invention, steam and oxygen are added to the air, resulting in decreased coke use, decreased iron use, and increased lead production. The air does not have to be preheated. The endothermicity of the steam reaction with coke is counteracted by the higher flame temperature of the coke combustion due to the oxygen enrichment of the combustion air. The role of steam in reducing coke use is readily understood by the fact that the reaction of steam with coke produces hydrogen which is a more effective reducer of lead oxide than is carbon monoxide. The mechanisms by which iron consumption is reduced are not understood in their entirety, although it is clear that the hydrogen does, in some complex fashion, affect the role of iron in its three functions, namely, 1) reduction of lead oxide, 2) oxidation to iron oxide in some other fashion to form a slag constituent, and 3) most importantly, reaction with the sulfur compounds to form iron sulfide, which is the principal component of matte.
The instant invention is primarily concerned with this type of lead recovery process.